Imaging device for moving a virtual image

ABSTRACT

There is provided an imaging device. The imaging device comprises a projection optic arranged to form a virtual image of an object. The imaging device further comprises a first diffuser positioned a first distance from the virtual projection optic and a second diffuser positioned a second distance from the virtual projection optic. The controller is arranged to control the first and second diffuser to make the real image visible on one of the diffusers.

FIELD OF THE INVENTION

The present disclosure relates to the field of imaging. Morespecifically, the present disclosure relates to the field of virtualimaging. The present disclosure also relate to the field of head-updisplays and viewing systems for head-up displays.

BACKGROUND

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well-known interferencetechniques to form a holographic recording, or “hologram”, comprisinginterference fringes. The “hologram” may be reconstructed byilluminating it with suitable light to form a holographicreconstruction, or replay image, representative of the original object.

It has been found that a holographic reconstruction of acceptablequality can be formed from a “hologram” containing only phaseinformation related to the original object. Such holographic recordingsmay be referred to as phase-only holograms. Computer-generatedholography may numerically simulate the interference process, usingFourier techniques for example, to produce a computer-generatedphase-only hologram. A computer-generated phase-only hologram may beused to produce a holographic reconstruction representative of anobject.

The term “hologram” therefore relates to the recording which containsinformation about the object and which can be used to form areconstruction representative of the object. The hologram may containinformation about the object in the frequency, or Fourier, domain.

It has been proposed to use holographic techniques in a two-dimensionalimage projection system. An advantage of projecting images usingphase-only holograms is the ability to control many image attributes viathe computation method—e.g. the aspect ratio, resolution, contrast anddynamic range of the projected image. A further advantage of phase-onlyholograms is that no optical energy is lost by way of amplitudemodulation.

A computer-generated phase-only hologram may be “pixellated”. That is,the phase-only hologram may be represented on an array of discrete phaseelements. Each discrete element may be referred to as a “pixel”. Eachpixel may act as a light modulating element such as a phase modulatingelement. A computer-generated phase-only hologram may therefore berepresented on an array of phase modulating elements such as a liquidcrystal spatial light modulator (SLM). The SLM may be reflective meaningthat modulated light is output from the SLM in reflection.

Each phase modulating element, or pixel, may vary in state to provide acontrollable phase delay to light incident on that phase modulatingelement. An array of phase modulating elements, such as a Liquid CrystalOn Silicon (LCOS) SLM, may therefore represent (or “display”) acomputationally-determined phase-delay distribution. If the lightincident on the array of phase modulating elements is coherent, thelight will be modulated with the holographic information, or hologram.The holographic information may be in the frequency, or Fourier, domain.

Alternatively, the phase-delay distribution may be recorded on akinoform. The word “kinoform” may be used generically to refer to aphase-only holographic recording, or hologram.

The phase delay may be quantised. That is, each pixel may be set at oneof a discrete number of phase levels.

The phase-delay distribution may be applied to an incident light wave(by illuminating the LCOS SLM, for example) and reconstructed. Theposition of the reconstruction in space may be controlled by using anoptical Fourier transform lens, to form the holographic reconstruction,or “image”, in the spatial domain. Alternatively, no Fourier transformlens may be needed if the reconstruction takes place in the far-field.

A computer-generated hologram may be calculated in a number of ways,including using algorithms such as Gerchberg-Saxton. TheGerchberg-Saxton algorithm may be used to derive phase information inthe Fourier domain from amplitude information in the spatial domain(such as a 2D image). That is, phase information related to the objectmay be “retrieved” from intensity, or amplitude, only information in thespatial domain. Accordingly, a phase-only holographic representation ofan object in the Fourier domain may be calculated.

The holographic reconstruction may be formed by illuminating the Fourierdomain hologram and performing an optical Fourier transform, using aFourier transform lens, for example, to form an image (holographicreconstruction) at a reply field such as on a screen.

FIG. 1 shows an example of using a reflective SLM, such as a LCOS-SLM,to produce a holographic reconstruction at a replay field location, inaccordance with the present disclosure.

A light source (110), for example a laser or laser diode, is disposed toilluminate the SLM (140) via a collimating lens (111). The collimatinglens causes a generally planar wavefront of light to become incident onthe SLM. The direction of the wavefront is slightly off-normal (e.g. twoor three degrees away from being truly orthogonal to the plane of thetransparent layer). The arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exiting wavefront (112). Theexiting wavefront (112) is applied to optics including a Fouriertransform lens (120), having its focus at a screen (125).

The Fourier transform lens (120) receives a beam of phase-modulatedlight exiting from the SLM and performs a frequency-space transformationto produce a holographic reconstruction at the screen (125) in thespatial domain.

In this process, the light—in the case of an image projection system,the visible light—from the light source is distributed across the SLM(140), and across the phase modulating layer (i.e. the array of phasemodulating elements). Light exiting the phase-modulating layer may bedistributed across the replay field. Each pixel of the hologramcontributes to the replay image as a whole. That is, there is not aone-to-one correlation between specific points on the replay image andspecific phase-modulating elements.

The Gerchberg Saxton algorithm considers the phase retrieval problemwhen intensity cross-sections of a light beam, I_(A)(x,y) andI_(B)(x,y), in the planes A and B respectively, are known and I_(A)(x,y)and I_(B)(x,y) are related by a single Fourier transform. With the givenintensity cross-sections, an approximation to the phase distribution inthe planes A and B, Φ_(A)(x,y) and Φ_(B)(x,y) respectively, is found.The Gerchberg-Saxton algorithm finds solutions to this problem byfollowing an iterative process.

The Gerchberg-Saxton algorithm iteratively applies spatial and spectralconstraints while repeatedly transferring a data set (amplitude andphase), representative of I_(A)(x,y) and I_(B)(x,y), between the spatialdomain and the Fourier (spectral) domain. The spatial and spectralconstraints are I_(A)(x,y) and I_(B)(x,y) respectively. The constraintsin either the spatial or spectral domain are imposed upon the amplitudeof the data set. The corresponding phase information is retrievedthrough a series of iterations.

A holographic projector may be provided using such technology. Suchprojectors have found application in head-up displays for vehicles.

The use of head-up displays in automobiles is becoming increasingpopular. Head-up displays are broken down in to two main categories,those which use a combiner (a free standing glass screen whose purposeis to reflect a virtual image in to the driver's line of sight) andthose which utilise the vehicle's windscreen to achieve the samepurpose.

FIG. 2 shows an example head-up display comprising a light source 206, aspatial light modulator 204 arranged to spatially modulate light fromthe light source with holographic data representative of an image forprojection, a Fourier transform optic 205, a diffuser 203, a freeformmirror 201, a windscreen 202 and a viewing position 207. FIG. 2 shows aso called “indirect view” system in which a real image of theholographic reconstruction is formed at a replay field on the diffuser203. A holographic reconstruction is therefore projected on the diffuser203 and may be viewed from viewing position 207 by focusing on thediffuser 203. The projected image is viewed via a first reflection offfreeform mirror 201 and a second reflection off windscreen 202. Thediffuser acts to increase the numerical aperture of the holographicsystem, fully illuminating the freeform mirrors thereby allowing thevirtual image to be viewed by a driver, for example.

Such display systems need to use a fixed diffuser or similar componentto increase the viewing angle. This diffuser serves as a key componentin the imaging system; its distance from the projection optic (normallya freeform mirror) determines the virtual image distance from theviewer's eye.

Alternatively, the holographic reconstruction may be viewed directly.Using “direct view” holography does enable information to be presentedin 3D, however as the name suggests direct view requires the viewer tolook at the hologram directly without a diffuser between the viewer andthe light source. This type of 3D display has a number of problems,firstly the current generation of phase modulators have a relativelysmall diffraction angle and therefore to create a sufficiently largeviewing area (eye-box) requires the use of complex and expensive opticsparts. Secondly and more importantly, this type of configurationrequired the viewer to be directly exposed to laser radiation. There arevery strict regulations surrounding the use of lasers and providing asufficiently robust safety system that will ensure that the eye is neverexposed to dangerous levels to laser radiation significantly increasesthe system complexity.

The use of a diffuser between the viewer and the projection enginemitigates both of the issues highlighted above and therefore a methodwhereby a diffuser could be employed in a display that offers multiplevirtual distances would offer a significant advantage.

The most obvious way to provide “depth” in a direct view system would beto mount the diffuser on a linear stage that was able to move backwardsand forwards within the focal length of the virtual imaging optic,thereby offering a mechanism to alter the virtual distance. However thistype of system is undesirable for use in a vehicle where build timeinstalled components are expected to offer a lifetime in excess of10,000 h. Additionally, a non-laser based projection engine would needto be able to refocus upon the diffuser as it changes location, addingadditional cost and complexity to the system

The present disclosure aims to provide a safe and robust viewing systemin which the perceived depth of the information being displayed may bevaried.

SUMMARY OF THE INVENTION

Aspects of an invention are defined in the appended independent claims.

There is disclosed a viewing device utilising a plurality of diffusersand a controller arranged to determine which one of the plurality ofdiffusers displays an object. In embodiments, the object is projectedonto the chosen diffuser. In embodiments, the diffusers are liquidcrystal devices which are switchable between a transmissive state inwhich they are effectively transparent to the projected light and ascattering state in which they display the object. In the scatteringstate, the diffuser acts like a screen.

Embodiments described a device in which an object is projected onto aplurality or array of diffusers but the object is only visible on one ofthe diffusers. Each diffuser is independently controlled to operate ineither a display (e.g. scattering) mode or a non-display (e.g.transparent) mode by, for example, controlling the voltage applied tothe diffuser. Optionally, the plurality of diffusers are closely-spacedor stacked. Notably, the diffusers are different distances from thevirtual imaging optic.

In an advantageous embodiment, the object is a holographicreconstruction projected using a holographic projector in whichprogrammable phase only lens data is applied to the holographic data toeffectively move the focal plane of the holographic reconstruction.Accordingly, small adjustments to focus may be easy made to compensationfor the different spatial positions of the plurality of diffusers.

Accordingly, a means for ensuring the object is focused, regardless ofwhich diffuser is activated for display, is provided.

Although embodiments describe “diffusers”, it may be understood that inexamples, the disclosed diffusers are operated in a non-diffusive modesuch as a transmissive mode. The diffusers may be considered as elementswith a selectable diffuse state or mode.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described to the accompanying drawings in which:

FIG. 1 is a schematic showing a reflective SLM, such as a LCOS, arrangedto produce a holographic reconstruction at a replay field location;

FIG. 2 shows a so-called “indirect view” holographic projector for ahead-up display of a vehicle;

FIG. 3 shows an example algorithm for computer-generating a phase-onlyhologram;

FIG. 4 shows an example random phase seed for the example algorithm ofFIG. 3;

FIG. 5 is a virtual imaging schematic;

FIG. 6 is multi-diffuser schematic in accordance with the presentdisclosure; and

FIG. 7 is a schematic of a LCOS SLM.

In the drawings, like reference numerals referred to like parts.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure relates to an improved so-called “indirect view”system in which a viewer views a virtual image of a holographicreconstruction. However, the disclosed indirect view system is equallysuitable for indirectly viewing any type of object visible on adiffuser. That is, although embodiments describe indirect view of aholographic reconstruction, the indirectly viewed “object” need notnecessarily be a holographic reconstruction. In other words, the presentdisclosure is equally applicable to other display systems such as moreconventional LED backlit liquid crystal display projectors and the like.Embodiments describe a method of computer-generating a hologram by wayof example only.

Holographically-generated 2D images are known to possess significantadvantages over their conventionally-projected counterparts, especiallyin terms of definition and efficiency.

Modified algorithms based on Gerchberg-Saxton have been developed—see,for example, co-pending published PCT application WO 2007/131650incorporated herein by reference.

FIG. 3 shows a modified algorithm which retrieves the phase informationψ[u,v] of the Fourier transform of the data set which gives rise to aknown amplitude information T[x,y] 362. Amplitude information T[x,y] 362is representative of a target image (e.g. a photograph). The phaseinformation ψ[u,v] is used to produce a holographic representative ofthe target image at an image plane.

Since the magnitude and phase are intrinsically combined in the Fouriertransform, the transformed magnitude (as well as phase) contains usefulinformation about the accuracy of the calculated data set. Thus, thealgorithm may provide feedback on both the amplitude and the phaseinformation.

The algorithm shown in FIG. 3 can be considered as having a complex waveinput (having amplitude information 301 and phase information 303) and acomplex wave output (also having amplitude information 311 and phaseinformation 313). For the purpose of this description, the amplitude andphase information are considered separately although they areintrinsically combined to form a data set. It should be remembered thatboth the amplitude and phase information are themselves functions of thespatial coordinates (x,y) for the farfield image and (u,v) for thehologram, both can be considered amplitude and phase distributions.

Referring to FIG. 3, processing block 350 produces a Fourier transformfrom a first data set having magnitude information 301 and phaseinformation 303. The result is a second data set, having magnitudeinformation and phase information ψn[u,v] 305. The amplitude informationfrom processing block 350 is set to a distribution representative of thelight source but the phase information ψ_(n)[u,v] 305 is retained. Phaseinformation 305 is quantised by processing block 354 and output as phaseinformation ψ[u,v] 309. Phase information 309 is passed to processingblock 356 and combined with the new magnitude by processing block 352.The third data set 307, 309 is applied to processing block 356 whichperforms an inverse Fourier transform. This produces a fourth data setR_(n)[x,y] in the spatial domain having amplitude information 311 andphase information 313.

Starting with the fourth data set, its phase information 313 forms thephase information of a fifth data set, applied as the first data set ofthe next iteration 303′. Its amplitude information R_(n)[x,y] 311 ismodified by subtraction from amplitude information T[x,y] 362 from thetarget image to produce an amplitude information 315 set. Scaledamplitude information 315 (scaled by α) is subtracted from targetamplitude information T[x,y] 362 to produce input amplitude informationη[x,y] 301 of the fifth data set for application as first data set tothe next iteration. This is expressed mathematically in the followingequations:

R _(n+1) [x, y]=F′{exp(iψ _(n) [u, v])}

ψ_(n) [u, v]=∠F{η·exp(i∠R _(n) [x, y])}

η=T[x, y]−α(|R _(n) [x, y]|−T[x, y])

Where:

F′ is the inverse Fourier transform;

F if the forward Fourier transform;

R is the replay field;

T is the target image;

∠ is the angular information;

Ψ is the quantized version of the angular information;

ε is the new target magnitude, ε≧0; and

α is a gain element ˜1.

The gain element a may be predetermined based on the size and rate ofthe incoming target image data.

In the absence of phase information from the preceding iteration, thefirst iteration of the algorithm uses a random phase generator to supplyrandom phase information as a starting point. FIG. 4 shows an examplerandom phase seed.

In a modification, the resultant amplitude information from processingblock 350 is not discarded. The target amplitude information 362 issubtracted from amplitude information to produce a new amplitudeinformation. A multiple of amplitude information is subtracted fromamplitude information 362 to produce the input amplitude information forprocessing block 356. Further alternatively, the phase is not fed backin full and only a portion proportion to its change over the last twoiterations is fed back. Accordingly, Fourier domain data representativeof an image of interest may be formed.

In summary, there is provided an improved imaging device comprising aplurality of controllable diffusers or selectively-diffusive elements.The diffusers are controlled such that an image is visible on only oneof the diffusers at any one point in time. By using a virtual projectionlens and positioning the diffusers at different distances from a virtualprojection optic, a variable position virtual image is formed.

FIG. 5 shows an embodiment of the present disclosure in which a virtualprojection optic 505 forms a virtual image 501 of an object 503. Theobject 503 is visible on a diffuser. The virtual image 501 is seen fromobservation plane 507.

The virtual image distance is set by placing the object inside the focallength of the imaging optic, the apparent virtual distance may then becalculated.

For the optical schematic shown above, the virtual image distance (i) isdetermined by the following equation:

$\begin{matrix}{i = \frac{1}{\frac{1}{f} - \frac{1}{o}}} & (1)\end{matrix}$

A linear change in the object distance causes a non-linear change in thevirtual distance.

The virtual image has to be sufficiently far from the eye so that theeye refocusing time from infinity, which is the normal focal length whendriving, to the display information is small thereby reducing the blindflight time. However, the virtual image distance must also besufficiently close so that the information presented to the driver isclearly legible. These two competing factors normally result in thevirtual image distance being configured such that the essential drivinginformation is presented at a distance of 1.5 to 3.5 m, optionally 2.5m, from the driver's eye.

The present disclosure provides a viewing device comprising a pluralityof diffusers and a controller arranged to control which diffuser theimage is visible on.

FIG. 6 shows an embodiment of the present disclosure in which thedistance from the viewer 620 to the virtual image 651, 652, 653, 654,formed by virtual projection optic 630, is changed by selecting whichdiffuser 601, 602, 603, 604 the image is visible on. In this embodiment,diffusers 601, 602, 603, 604 are substantially parallel and stackedtogether. In this embodiment, the diffusers are spaced by glasssubstrate 610. If the image is visible on diffuser 601, the virtualimage 651 will appear at a first depth to viewer 620. If the image isvisible on diffuser 602, the virtual image 652 will appear at a seconddepth to viewer 620, and so on.

There is therefore provided an imaging device comprising: a projectionoptic arranged to form a virtual image of a real image; a first diffuserpositioned a first distance from the virtual projection optic; a seconddiffuser positioned a second distance from the virtual projection optic;and a controller arranged to control the first and second diffusers tomake the real image visible on one of the diffusers. That is, thecontroller is arranged to control which diffuser the real image isvisible on.

Accordingly, the effective position of the virtual image may be changedby selected which diffuser the real image is visible, or “displayed”,on.

There is also provided a method of moving a virtual image using aplurality of diffusers, the method comprising: forming a virtual imageof a real image visible on a diffuser using a projection optic;controlling whether the real image is visible on a first diffuserpositioned a first distance from the projection optic or a seconddiffuser positioned a second distance from the projection optic.

In an embodiment, the real image is a holographic reconstruction but aspreviously described, the real image may be projected onto thediffuser/s by any means.

In an embodiment, each diffuser is independently switchable between ascattering mode and a transmissive mode. A diffuser operating in thetransmissive mode will transmit the projected object but a diffuseroperating in the scattering mode will effectively “display” the object.That is, the (real) image will be visible on the diffuser operating inthe scattering mode. By stacking a plurality of diffusers together andarranging them such that each is at a different distance from thevirtual projection optic, the parameter “o” in equation 1 may be varied.Accordingly, the distance from the observation plane to the viewedvirtual image is changed. It may therefore be understood that theperceived depth of the displayed information may be changed by selectingthe diffuser.

In an embodiment, the controller is arranged to operate no more than onediffuser in the scattering mode at any one point in time. That is, onediffuser is operated in the scattering mode and all other diffusers areoperated in the transmissive mode.

In an embodiment, to enable an eye safe, low complexity, variabledistance head up display, liquid crystal devices are used that canswitch between a scattering and transmissive state. That is, in anembodiment, the first and/or second diffusers comprise liquid crystalsin which a light scattering state may be selectively-induced.

By sandwiching a number of these devices together the effective positionof the diffuser may be simply by choosing which device to energise in toa scattering state.

Light scattering states can be induced in thin liquid crystal layers bya number of mechanisms. In each case the refractive index of the liquidcrystal varies from point to point with a magnitude and spatialfrequency sufficiently close to the wavelength of light so as to resultin strong scattering.

Some of these liquid crystal electro-optic effects are static and someare dynamic (consisting of turbulent motion). The static effects may betransient (only present when a voltage is maintained), bistable (asingle scattering state that can be switched on and off with a voltagepulse) or multistable (a number of different stable scattering statesthat can be switched on and off with voltage pulses. That is, in anembodiment, the scattering state is selectively-induced by voltage.

Embodiments use liquid crystals selected from the group comprising:

(1) Cholesteric liquid crystals (also called chiral nematic phases) witha suitably small cholesteric pitch can be driven into transparent andlight scattering states by dielectric re-orientation. Polymer materialsmay be added to these materials to stabilise the textures. The texturesare static and can exhibit bi-stability, (but not multi-stability). Formore information concerning this type of liquid crystal, the reader isreferred to Gruebel. W., U. Wolff., and H. Kruber., “Electric fieldinduce texture changes in certain nematical cholesteric liquid crystalmixtures”., Mol. Cryst. Liq. Cryst, Vol. 24, 1973, pp 103-109 and V. G.Chigrinov, “Liquid Crystal devices, Physics and Applications”, ISBN0-89006-895-4, Published by Artech House, 1999, pp 134-148.

(2) Films of nematic liquid droplets in a polymer matrix (polymerdispersed liquid crystal or “PDLCs”) can exhibit light scattering andcan be switched into a clear state by dielectric re-orientation. This isa static texture and relaxes back to a clear state on removal of thedrive voltage, i.e. PDLCs are not usually bi-stable. Some bi-stabilitycan be induced by using a chiral nematic liquid crystal (i.e. acholesteric liquid crystal) instead of the nematic phase in thedroplets. For more information concerning this type of liquid crystal,the reader is referred to Coates D., “Polymer dispersed LiquidCrystals”, J. Mater. Chem., Vol. 5, No. 12., 1994, pp 2063-2072 andDoane, J. W., et al., “Wide-angle View PDLC Displays”. SID '90 Digest,1990, pp 224-226.

(3) Dynamic scattering can also be electro-chemically induced in theliquid crystal smectic A phase, which are more ordered that nematicphases. The application of a low frequency voltage produces a turbulentdynamic scattering state resembling that occurring in nematic liquidcrystals. However when the voltage is removed, the scattering state doesnot relax back to clear state, but remains as a semi-permanent statictexture. It can however then be removed by applying a higher frequencyvoltage (>1 KHz). This scattering state is ‘multi-stable’ in thatdifferent degrees of scattering can be induced and they are all stablein the absence of voltage. High voltages (around 100 V) are required forsmectic dynamic scattering. For more information concerning this type ofliquid crystal, the reader is referred to: D. Coates, W. A. Crossland,J. H. Morrissy, and B. Needham, J. Phys. D. 11, 1 (1978); and CrosslandW. A., Davey A. B., Chu D., Clapp T. V., “Smectic A Memory Displays”, inHandbook of Liquid Crystals: 7 Volume Set, Second Edition. Edited by J.W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. Gleeson, and P.Raynes. . Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA. Chapter 7,pp 1-39.

The inventors have identified which of these liquid crystals aresuitable in accordance with the present disclosure. These possibilitiesare summarised in the table below.

TABLE 1 Bi-stability Multi-stability Switchable of scattering ofscattering diffusers state state Cholesteric liquid crystals Yes Yes NoPDLCs Yes Possible No Smectic dynamic Yes Yes Yes scattering

In an embodiment, the first and/or second diffusers comprise cholestericliquid crystals. In another embodiment, the first and/or seconddiffusers comprise polymer dispersed liquid crystals. In a furtherembodiment, the first and/or second diffusers comprise smectic-A liquidcrystals.

As shown in FIG. 6, in an embodiment, the first and second diffuser aresubstantially parallel and/or positioned on a common optical axis.

Although FIG. 6 shows an arrangement of four diffusers, it may beunderstand that any number of diffusers may be employed depending on theresolution required. That is, in an embodiment, the device furthercomprises a plurality of further diffusers positioned at differentdistances from the projection optic. All the diffusers may besubstantially parallel and/or positioned on a common optical axis. In anembodiment, the diffusers are on a common optical axis with the virtualprojection optic.

In an optional embodiment, an image is visible on the chosen diffuserbecause the diffusor is scattering and the image is projected onto thediffuser by a projector. In an embodiment, the image is projected ontothe diffuser by a holographic projector and the object is a holographicreconstruction of a predetermined object. An example holographicprojector has been previous described in, for example, WO 2013/153354incorporated herein by reference.

In an embodiment, there is therefore provided a display systemcomprising the previous described imaging device and a holographicprojector comprising a spatial light modulator arranged to apply aphase-delay distribution to incident light, wherein the phase-delaydistribution comprises phase-only data representative of a lens andphase-only data representative of the object.

It is known in the art how a phase-only programmable lens may becombined with phase-only object data such that, when reconstructed byreverse Fourier transform (e.g. optically), a focused holographicreconstruction is formed at a chosen depth of replay field. Inembodiments, the data is combined by simple vector addition.

In an embodiment, the holographic projector therefore further comprisesa Fourier transform optic arranged to perform an optical Fouriertransform of phase modulated light received from the spatial lightmodulator to form the object.

The disclosed method of moving a virtual image may therefore furthercomprise: applying a phase-delay distribution to incident light whereinthe phase-delay distribution comprises phase-only data representative ofa lens and phase-only data representative of the object; performing anoptical Fourier transform of phase modulated light received from aspatial light modulator to form the object; and selecting the focallength of the Fresnel lens so that the image is substantially focused onthe diffuser on which the image is visible.

As the diffusion plane may be changed, the distance from the projectionsystem will also have changed, this may defocus and therefore degradethe quality of the displayed image. However, advantageously, where theabove system in employed in conjunction with the phase only holographicprojector, this problem is overcome by combining the hologram with aphase only representation of an appropriate focal length lens. Innon-laser based systems an adjustable autofocus may need to be employedadding complexity to the projection engine.

That is, in an advantageous embodiment, the holographic projector isfurther arranged to select the focal length of the phase only lens sothat the image is substantially focused on the diffuser on which theimage is visible.

In a further embodiment, the Fourier transform optic utilised by theholographic projector is not a physical optic but, instead, a furtherphase only lens implemented using the same holographic techniques.

In embodiment, the display system is a head-up display although it maybe understood the disclosed imaging device is equally applicable toother display systems and projection systems.

Example System

The system outlined in FIG. 6, shows a system with four liquid crystaldiffusers, separated by glass windows. For a system employing a virtualimaging lens with 200 mm back focal length and 5 mm thick windowseparating each of the switchable diffusers, the following virtualdistances would be possible.

TABLE 2 Diffuser Diffuser location Virtual image distance 1 181 mm 2.02m 2 187 mm 2.88 m 3 192 mm 4.80 m 4 197 mm 13.13 m 

Although examples show four diffusers linearly spaced, the spacing couldbe non-linear, and the number of diffusers could be altered dependingupon the desired distance resolution.

Advantageously, the manufacture and assembly of these liquid crystaldiffusers is low cost, due to the absence of pixelated circuitry and theassociated drivers, that the use of multiple diffusers in a packagewould not be cost prohibitive.

It can be understood that a head-up display may display a variety ofinformation as known in the art. Holograms corresponding to all thepossible displays may be therefore be pre-calculated and stored in arepository, or calculated in real-time. In an embodiment, the projectorfurther comprises a repository of Fourier domain data representative ofa plurality of 2D images.

Embodiments described herein relate to Fourier holography by way ofexample only. The present disclosure is equally applicable to Fresnelholography in which Fresnel transform is applied during calculation ofthe hologram.

The quality of the reconstructed hologram may be affect by the so-calledzero order problem which is a consequence of the diffractive nature ofthe reconstruction. Such zero-order light can be regarded as “noise” andincludes for example specularly reflected light, and other unwantedlight from the SLM.

This “noise” is generally focussed at the focal point of the Fourierlens, leading to a bright spot at the centre of a reconstructedhologram. Conventionally, the zero order light is simply blocked outhowever this would clearly mean replacing the bright spot with a darkspot.

Alternatively and angularly selective filter could be used to removeonly the collimated rays of the zero order. Other methods of managingthe zero order may also be used.

Whilst embodiments described herein relate to displaying one hologramper frame, the present disclosure is by no means limited in this respectand more than one hologram may be displayed on the SLM at any one time.

For example, embodiments implement the technique of “tiling”, in whichthe surface area of the SLM is further divided up into a number oftiles, each of which is set in a phase distribution similar or identicalto that of the original tile. Each tile is therefore of a smallersurface area than if the whole allocated area of the SLM were used asone large phase pattern. The smaller the number of frequency componentin the tile, the further apart the reconstructed pixels are separatedwhen the image is produced. The image is created within the zerothdiffraction order, and it is preferred that the first and subsequentorders are displaced far enough so as not to overlap with the image andmay be blocked by way of a spatial filter.

As mentioned above, the image produced by this method (whether withtiling or without) comprises spots that form image pixels. The higherthe number of tiles used, the smaller these spots become. If one takesthe example of a Fourier transform of an infinite sine wave, a singlefrequency is produced. This is the optimum output. In practice, if justone tile is used, this corresponds to an input of a single cycle of asine wave, with a zero values extending in the positive and negativedirections from the end nodes of the sine wave to infinity. Instead of asingle frequency being produced from its Fourier transform, theprinciple frequency component is produced with a series of adjacentfrequency components on either side of it. The use of tiling reduces themagnitude of these adjacent frequency components and as a direct resultof this, less interference (constructive or destructive) occurs betweenadjacent image pixels, thereby improving the image quality.

Preferably, each tile is a whole tile, although it is possible to usefractions of a tile.

Embodiments relate to variants of the Gerchberg-Saxton algorithm by wayof example only.

The skilled person will understand that the improved method disclosedherein is equally applicable to the calculation of a hologram used toform a three-dimensional reconstruction of an object.

Equally, the present disclosure is not limited to projection of amonochromatic image.

A colour 2D holographic reconstruction can be produced and there are twomain methods of achieving this. One of these methods is known as“frame-sequential colour” (FSC). In an FSC system, three lasers are used(red, green and blue) and each laser is fired in succession at the SLMto produce each frame of the video. The colours are cycled (red, green,blue, red, green, blue, etc.) at a fast enough rate such that a humanviewer sees a polychromatic image from a combination of the threelasers. Each hologram is therefore colour specific. For example, in avideo at 25 frames per second, the first frame would be produced byfiring the red laser for 1/75th of a second, then the green laser wouldbe fired for 1/75th of a second, and finally the blue laser would befired for 1/75th of a second. The next frame is then produced, startingwith the red laser, and so on.

An alternative method, that will be referred to as “spatially separatedcolours” (SSC) involves all three lasers being fired at the same time,but taking different optical paths, e.g. each using a different SLM, ordifferent area of a single SLM, and then combining to form the colourimage.

An advantage of the frame-sequential colour (FSC) method is that thewhole SLM is used for each colour. This means that the quality of thethree colour images produced will not be compromised because all pixelson the SLM are used for each of the colour images. However, adisadvantage of the FSC method is that the overall image produced willnot be as bright as a corresponding image produced by the SSC method bya factor of about 3, because each laser is only used for a third of thetime. This drawback could potentially be addressed by overdriving thelasers, or by using more powerful lasers, but this would require morepower to be used, would involve higher costs and would make the systemless compact.

An advantage of the SSC (spatially separated colours) method is that theimage is brighter due to all three lasers being fired at the same time.However, if due to space limitations it is required to use only one SLM,the surface area of the SLM can be divided into three parts, acting ineffect as three separate SLMs. The drawback of this is that the qualityof each single-colour image is decreased, due to the decrease of SLMsurface area available for each monochromatic image. The quality of thepolychromatic image is therefore decreased accordingly. The decrease ofSLM surface area available means that fewer pixels on the SLM can beused, thus reducing the quality of the image. The quality of the imageis reduced because its resolution is reduced.

In embodiments, the SLM is a Liquid Crystal over silicon (LCOS) device.LCOS SLMs have the advantage that the signal lines, gate lines andtransistors are below the mirrored surface, which results in high fillfactors (typically greater than 90%) and high resolutions.

LCOS devices are now available with pixels between 2.5 μm and 15 μm.

The structure of an LCOS device is shown in FIG. 7.

An LCOS device is formed using a single crystal silicon substrate (802).It has a 2D array of square planar aluminium electrodes (801), spacedapart by a gap (801 a), arranged on the upper surface of the substrate.Each of the electrodes (801) can be addressed via circuitry (802 a)buried in the substrate (802). Each of the electrodes forms a respectiveplanar mirror. An alignment layer (803) is disposed on the array ofelectrodes, and a liquid crystal layer (804) is disposed on thealignment layer (803). A second alignment layer (805) is disposed on theliquid crystal layer (404) and a planar transparent layer (806), e.g. ofglass, is disposed on the second alignment layer (805). A singletransparent electrode (807) e.g. of ITO is disposed between thetransparent layer (806) and the second alignment layer (805).

Each of the square electrodes (801) defines, together with the overlyingregion of the transparent electrode (807) and the intervening liquidcrystal material, a controllable phase-modulating element (808), oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels (801 a). By control of the voltageapplied to each electrode (801) with respect to the transparentelectrode (807), the properties of the liquid crystal material of therespective phase modulating element may be varied, thereby to provide avariable delay to light incident thereon. The effect is to providephase-only modulation to the wavefront, i.e. no amplitude effect occurs.A major advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer can be half the thickness than would benecessary if a transmissive device were used. This greatly improves theswitching speed of the liquid crystal (a key point for projection ofmoving video images). A LCOS device is also uniquely capable ofdisplaying large arrays of phase only elements in a small aperture.Small elements (typically approximately 10 microns or smaller) result ina practical diffraction angle (a few degrees) so that the optical systemdoes not require a very long optical path.

It is easier to adequately illuminate the small aperture (a few squarecentimetres) of a LCOS SLM than it would be for the aperture of a largerliquid crystal device. LCOS SLMs also have a large aperture ratio, therebeing very little dead space between the pixels (as the circuitry todrive them is buried under the mirrors). This is an important issue tolowering the optical noise in the replay field.

Using a silicon backplane has the advantage that the pixels areoptically flat, which is important for a phase modulating device.

Whilst embodiments relate to a reflective LCOS SLM, the skilled personwill understand that any SLM can be used including transmissive SLMs.

The invention is not restricted to the described embodiments but extendsto the full scope of the appended claims.

1. An imaging device, comprising: a projection optic arranged to form avirtual image of a real image; a first diffuser positioned a firstdistance from the virtual projection optic; a second diffuser positioneda second distance from the virtual projection optic; and a controllerarranged to control the first and second diffusers to make the realimage visible on one of the diffusers.
 2. The imaging device of claim 1,wherein the real image is a holographic reconstruction.
 3. The imagingdevice of claim 1, wherein each diffuser is independently switchablebetween a scattering mode and a transmissive mode.
 4. The imaging deviceof claim 3, wherein the controller is arranged to operate no more thanone diffuser in the scattering mode at any one point in time.
 5. Theimaging device of claim 1, wherein at least one of the first or seconddiffusers comprises liquid crystals in which a light scattering statemay be selectively-induced.
 6. The imaging device of claim 5, whereinthe scattering state is selectively-induced by voltage.
 7. The imagingdevice of claim 1, wherein at least one of the first or second diffuserscomprises cholesteric liquid crystals.
 8. The imaging device of claim 1,wherein at least one of the first or second diffusers comprises polymerdispersed liquid crystals.
 9. The imaging device of claim 1, wherein atleast one of the first or second diffusers comprises smectic-A liquidcrystals.
 10. The imaging device of claim 1, wherein the first andsecond diffusers are substantially parallel.
 11. The imaging device ofclaim 1, further comprising a plurality of further diffusers positionedat different distances from the projection optic.
 12. A display system,comprising: the imaging device of claim 1; and a holographic projectorcomprising a spatial light modulator arranged to apply a phase-delaydistribution to incident light, wherein the phase-delay distributioncomprises phase-only data representative of a lens and phase-only datarepresentative of the real image.
 13. The display system of claim 12,wherein the holographic projector further comprises a Fourier transformoptic arranged to perform an optical Fourier transform of phasemodulated light received from the spatial light modulator to form thereal image.
 14. The display system of claim 12, wherein the holographicprojector is further arranged to select the focal length of the lens sothat the object is substantially focused on the diffuser on which theobject is visible.
 15. The display system of claim 12, wherein thedisplay system is a head-up display.
 16. A method of moving a virtualimage using a plurality of diffusers, the method comprising: forming avirtual image of a real image visible on a first diffuser or a seconddiffuser of the plurality of diffusers using a projection optic;controlling whether the real image is visible on the first diffuserpositioned a first distance from the projection optic or the seconddiffuser positioned a second distance from the projection optic.
 17. Themethod of claim 16, further comprising: applying a phase-delaydistribution to incident light wherein the phase-delay distributioncomprises phase-only data representative of a lens and phase-only datarepresentative of the object; performing an optical Fourier transform ofphase modulated light received from a spatial light modulator to formthe object; and selecting the focal length of the lens so that theobject is substantially focused on the diffuser on which the object isvisible.
 18. (canceled)
 19. The imaging device of claim 1, wherein thefirst and second diffusers are positioned on a common optical axis. 20.The display system as claimed of claim 12, wherein, when the displaysystem is installed in a vehicle, the display system is configured toform the virtual image at a distance between 1.5 m and 3.5 m from adriver's eye.
 21. A vehicle comprising the display system of claim 20installed in the vehicle.